Method to create a permanent plug by inducing movement in caprock

ABSTRACT

A process is directed at plugging a wellbore, wherein the wellbore has a portion extending at least partially through a caprock composed of salt rock. The process includes applying a barrier in the wellbore at or above the caprock. Thereafter, the process includes a step of promoting creep such that the salt rock in the caprock creeps into the wellbore to create a salt-rock plug in the wellbore, and hence, closes off the portion of the wellbore above the salt-rock plug from fluid flow communication with the portion of the wellbore below the salt-rock plug.

FIELD

The present disclosure relates generally to the field of cementing a wellbore, and more specifically, to creating plugs in wellbores, such as for in abandonment operations.

BACKGROUND

When drilling a wellbore that penetrates one or more subterranean earth formations, it may be advantageous or necessary to create a hardened plug in the borehole. Such plugs are used for abandonment of the well, wellbore isolation, wellbore stability, or kickoff procedures. There are a number of systems used to create the hardened plug.

For example, a cement plug may be set in a borehole by pumping a volume of spacer fluid compatible with the drilling mud and cement slurry into the work string. Then, a predetermined volume of cement slurry is pumped behind the spacer fluid. The cement slurry travels down the work string and exits into the wellbore to form a plug.

Plug formulations are important for ensuring that a plug maintains long-term mechanical integrity when used for well abandonment and kick-off operations. Problems with plugs can result from excessive shrinkage or expansion of the plug material during curing, which causes debonding or cracks. Additionally, unwanted production from below the plug after abandonment can exert load on the plug, which can degrade its integrity. For example, the unwanted production can release CO₂, or other corrosive gases, which can cause mechanical damage to cement plugs.

Accordingly, it is desirable to develop improvements in wellbore plugging methods to form more durable and stable plugs.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings included with this application illustrate certain aspects of the embodiments described herein. However, the drawings should not be viewed as exclusive embodiments. The subject matter disclosed herein is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will be evident to those skilled in the art with the benefit of this disclosure.

FIG. 1 is a graphical illustration of the time for a prescribed amount of wellbore closure. FIG. 1 illustrates time for well closure time versus mud weight.

FIG. 2 schematically illustrates the initial state of a wellbore in one embodiment of a process using the principles of this disclosure for plugging a wellbore.

FIG. 3 schematically illustrates a later state of the wellbore in the embodiment of FIG. 2 .

FIG. 4 schematically illustrates a second embodiment of a process using the principles of this disclosure for plugging a wellbore.

DETAILED DESCRIPTION

The present disclosure may be understood more readily by reference to this detailed description, including the figures. For simplicity and clarity of illustration, where appropriate, reference numerals may be repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts may have been exaggerated to better illustrate details and features of the present disclosure.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to ... ”. Reference to up or down will be made for purposes of description with "up," "upper," "upward," or "upstream" meaning toward the surface of the wellbore and with "down," "lower," "downward," or "downstream" meaning toward the terminal end of the well, regardless of the wellbore orientation.

As used herein, “caprock” or “cap rock” is any nonpermeable formation that prevents oil, gas or water migrating to the surface. In particular, in this disclosure, caprock or cap rock refers to salt domes or salt layers that are composed of salt rock as defined below.

As used herein, “cased hole” refers to that portion of a wellbore that has had a casing or liner placed and cemented to the wellbore.

As used herein, “open-hole” refers to the uncased portion of a well. All wells, at least when first drilled, have open-hole sections. Generally, casing is set and cemented in place to isolate a formation from the rest of the wellbore. A well can be entirely open-hole, partially cased, or entirely cased. This disclosure focuses on open-hole and partially cased wellbores.

As used herein, “salt rock” or “salt layer” refers to underground or subterranean salt layers or salt domes. Such salt structures are formed by different minerals. For instance, rock salt also called halite salt crystal, is made of NaCl crystal; however, most such structures are impure comprising a mixture of salts and other components, such as gypsum and anhydrite. Other examples of salt structures are Tachyhydrite, Carnallite, etc. As used herein, “salt rock” or “salt layers” will refer to structures that are greater than 50% by weight of salt crystal, and more typically, at least 60%, at least 75%, at least 80% or even at least 90% by weight of salt crystal. Particularly preferred is salt rock comprising at least 50% by weight salt, or at least 60%, at least 75%, at least 80%, or at least 90% by weight salt. Due to the unique physical and chemical properties of salt rock, such as its crystal lattice, high thermal conductivity and high solubility in water, it deforms distinctively in underground and surface environments compared with other rocks. Instability of salt rock is also given by its low viscosity, which allows salt rock to flow as a fluid.

As used herein, the term “strain” or “deformation” means a measure of the extent to which a body of material is deformed and/or distorted when it is subjected to a stress-inducing force. “Stress-Inducing Force” refers to an action of at least one force, load and/or constraint on a body of material that tends to strain the body. Examples of the body’s deformation or distortion can include, without limitation, changes in the body’s length (e.g., linear strain), volume (e.g., bulk strain) and/or a lateral displacement between two substantially parallel planes of material within the body (e.g., shear strain).

“Stress” is a measure of inter-particle forces arising within a body of material resisting deformation and/or distortion, in response to a stress-inducing force applied to the body, as particles within the body of material work to resist separation, compression and/or sliding.

This disclosure is directed towards creating a permanent plug in a wellbore such as for the purposes of abandoning the well. The conventional approach to create a permanent plug to abandon the well involves placing a foreign material like cement, resin, etc., adjacent to caprock. In this state, there is a foreign material surrounded by caprock. Such a plug barrier’s efficacy is a subject of scrutiny because a foreign material has taken the place of caprock. Potential risks are loss of bond between foreign material and caprock, degradation or mechanical failure of foreign material and others.

This disclosure is based on the recognition that the most ideal approach is to restore the caprock continuum. The proposed method creates an operational process in which a temporary barrier, often an annular barrier (between a work string and open—or previously cased—hole), is set in the wellbore. This barrier could be a packer such as RTTS^(tm) packer or eZSVB^(tm) packer, both available from Halliburton Energy Services. Below this temporary barrier, favorable conditions are present for salt rock to creep and form a permanent barrier. Upon completion of such a process, salt forms a competent caprock restoring pre-drilling state of the caprock continuum. This method is applicable when the caprock contains enough salt to induce creep. Generally, this method is applicable to caprock containing greater than 50% by weight salt crystals, and more typically, at least 60%, at least 75%, at least 80% or even at least 90% by weight salt crystals.

Salts have properties that make them an ideal caprock. They are practically nonporous and impermeable, thick and long and have high enough strength to isolate reservoir pressures. The method uses the special ability of salt to deform permanently under the action of differential stress, often termed “creep”.

The constitutive equation that governs this creep process is usually empirical in nature. An example equation is shown below.

$\overset{˙}{\varepsilon} = A_{1}\exp\left( \frac{- Q_{1}}{RT} \right)\left( \frac{S_{2}}{S_{2}^{o}} \right)^{n_{1}} + A_{2}\exp\left( \frac{- Q_{2}}{RT} \right)\left( \frac{S_{2}}{S_{2}^{o}} \right)^{n_{2}}$

Where,

$S_{2} = \sqrt{\frac{1}{2}\left( {\left( {\sigma_{1} - \sigma_{2}} \right)^{2} + \left( {\sigma_{2} - \sigma_{3}} \right)^{2} + \left( {\sigma_{3} - \sigma_{1}} \right)^{2}} \right)}$

-   ε̇ — Second invariant of strain rate -   S₂ - Second invariant of stress rate -   Q₁, Q₂ —Activation energies -   n₁, n₂ - Power law coefficients -   A_(1,) A₂ — Creep rate coefficients -   S₂⁰ -   — Reference deviatoric stress -   σ_(n) - Creep effective stress

Alternate equation forms are also possible. The parameters of such equations are extracted by performing a numerical match of creep rate with experiments. These experiments in turn can be downhole measurements of creep vs. time or triaxial creep tests performed on extracted salt cores.

A salt rock described with a creep equation like the one above will deform when subjected to differential stress. Generally, the differential stress and temperature increases with depth and thus results in higher creep rate. The current disclosure exploits this behavior to let the salt creep and close an existing borehole, thus forming a caprock continuum.

FIG. 1 shows the time taken for a pre-determined amount of borehole closure due to salt creep at a specific depth in the wellbore. FIG. 1 illustrates the wellbore closure time versus mud weight. In the study of FIG. 1 , the mud weight was lower than the salt rock density. Thus, decreasing mud weight means that the differential stress at the wellbore mouth is increasing. As can be seen from numerical analysis, a lower mud weight (i.e., a higher differential stress) results in an earlier closure of borehole. A similar effect is observed with temperature. When the temperature of near wellbore increases, the salt creeps faster. By altering the two drivers, downhole pressure, and temperature, it is possible to tune the creep rate and thus the time for closure of wellbore. Other means, like chemical, of activating the creep process may also be used.

Turning now to FIGS. 2 and 3 , a process for plugging a wellbore using the principles of this disclosure will now be described. As illustrated, a wellbore 10 has a portion 12 extending at least partially through a caprock 14 composed of salt rock. This portion is open-hole. While the entire borehole can be open-hole, as illustrated, the upper portion (above the caprock) is a cased hole having casing 16 cemented by cement 18 to the wellbore wall.

As a first step, a temporary annular barrier 20 is created above the salt rock. In an open-hole wellbore, the temporary annular barrier will be in the open-hole. In a cased hole, the temporary barrier can be created inside the casing 16 or can be in the open-hole below casing 16. Typically, the temporary barrier only needs to last until the salt caprock is formed to close off the wellbore (as described below). Generally, the formation of the caprock (and the time the barrier needs to last) is less than one month, more typically less than two weeks, or less than one week, and this period can be one to five days, or even one to three days.

The temporary annular barrier 20 can be formed using a bridge plug or other mechanical elements. Optionally, the temporary annular barrier 20 can be made of elastomeric materials instead of mechanical barriers. However, the formed temporary barrier includes a path 22 allowing access to the wellbore below the barrier. The path allows regulation of the temperature and pressure below the temporary barrier. Thus, pressure and temperature can be altered through the path such that the differential stress in the caprock zone is regulated, as further discussed below.

For example, a pipe 24 can be placed through the temporary barrier before or after placement of the barrier. Generally, the path through the barrier allows fluid to flow to and from the region below the barrier and at the caprock. Accordingly, in some embodiments, the path includes a pipe-in-pipe assembly such that fluid can be introduced through the central pipe and then returned through an annulus between the central pipe and outer pipe.

In the second step, creep (represented by arrows 26) is promoted such that the salt rock in the caprock creeps into the wellbore. For example, creep can be promoted by regulating the pressure and/or temperature below the temporary barrier. For example, the fluid can be introduced through the path in the temporary barrier and pressure can be regulated through changing the density and/or surface pressure of the fluid, and/or temperature at the caprock can be regulated by changing the temperature of the fluid in the pipe. The regulation of pressure and temperature allows creation of differential stress. Under conditions of high differential stress, the salt rock creeps in the direction indicated by arrows 26 in FIG. 2 . This results in closure of wellbore, thus forming a continuum of salt as caprock.

Typically, pressure and temperature are regulated to maintain maximum possible differential stress in the salt body adjacent to wellbore. Under these conditions, the creep rate is the highest causing the wellbore to close over time. This step is generally continued till a complete wellbore closure occurs, as shown in FIG. 3 . In this state, the caprock continuum is restored to its pre-drilling state by newly created caprock 28.

In some embodiments, another plug barrier (not shown) can be placed above the newly created caprock 28. This step will avoid the possibility of future undesirable creep due to changes in wellbore temperature or pressure above the salt plug due to events like a side track through the same wellbore, etc. This plug barrier or upper foreign-material barrier will typically be formed from cement or resin.

In other embodiments, instead of restoring the caprock continuum, a mixed foreign material and salt rock plug can be formed. In these embodiments, a foreign-material plug (not shown) is introduced into the portion of the wellbore extending at least partially through the caprock. Typically, this foreign material plug is made of cement or resin. The foreign-material plug is introduced prior to promoting creep. After introduction, creep is promoted; for example, by creating a differential stress state in salt such that the bond between the foreign material and salt is strengthened. This can be done by altering the differential stress near the salt rock such that it creeps radially inward and compress fits to the foreign material.

In some embodiments, lower foreign-material plug 30 may have to be placed below or within a portion of the caprock zone as illustrated in FIG. 4 . Placement of plug 30 avoids risk of inflow or losses of fluids below the caprock during pressure or temperature changes used to induce creep. Generally, the lower foreign-material plug will be placed before promoting creep, and more typically, before applying the barrier 20 in the wellbore at or above the caprock. As will be realized, if applied at the caprock, plug 30 will be below barrier 20 and generally towards the bottom of the caprock. Plug 30 typically is formed from cement or resin.

The above disclosure is exemplified by processes defined by the following numbered paragraphs.

-   1. A process for plugging a wellbore having a portion extending at     least partially through a caprock composed of salt rock, the process     comprising: applying a barrier in the wellbore at or above the     caprock; and after applying the barrier, promoting creep such that     the salt rock in the caprock creeps into the wellbore to create a     salt-rock plug in the wellbore to seal across the entire wellbore,     and hence, close off the portion of the wellbore above the salt-rock     plug from fluid flow communication with the portion of the wellbore     below the salt-rock plug. -   2. The process of paragraph 1, wherein creep is promoted by     regulating the pressure or temperature below the barrier. -   3. The process of paragraph 2, wherein the pressure or temperature     are regulated to maintain maximum possible differential stress in     the caprock. -   4. The process of paragraphs 2 or 3, wherein pressure or temperature     is regulated by introducing a fluid into a portion of the wellbore     extending through the caprock. -   5. The process of paragraph 4, wherein pressure is regulated by     changing the density or surface pressure of the fluid. -   6. The process of paragraph 4, wherein temperature is regulated by     changing the temperature of the fluid. -   7. The process of any of paragraphs 4 to 6, further comprising     inserting a pipe through the barrier, and wherein a fluid is     introduced through the pipe into the portion of the wellbore     extending through the caprock. -   8. The process of any of paragraphs 1 to 7, wherein the barrier is a     packer. -   9. The process of any of paragraphs 1 to 8, wherein the step of     promoting creep is continued until a complete wellbore closure     occurs such that the caprock continuum is restored to the state     prior to drilling of the wellbore. -   10. The process of any of paragraphs 1 to 9, wherein after the step     of promoting creep, the process further comprises placing a plug     barrier above the salt-rock plug. -   11. The process of any of paragraphs 1 to 10, wherein prior to     promoting creep, the process comprises introducing an upper     foreign-material plug into the portion of the wellbore extending at     least partially through the caprock, such that during the step of     promoting creep, salt rock creeps radially inward and compress fits     to the foreign-material plug. -   12. The process of paragraph 11, wherein the upper foreign-material     plug comprises cement, resin or both. -   13. The process of any one of paragraphs 1 to 12, wherein prior to     the promoting of creep, the process further comprising introducing a     lower foreign-material plug into the wellbore in a portion of the     wellbore below the caprock. -   14. The process of paragraph 13, wherein the lower foreign-material     plug comprises cement, resin or both. -   15. The process of any of paragraphs 1 to 13, wherein the caprock     comprises at least 50% by weight salt crystals, or optionally at     least 60%, at least 70%, at least 80% or at least 90% by weight salt     crystals. -   16. The process of any of paragraphs 1 to 13, wherein the caprock     comprises at least 50% by weight salt, or optionally at least 60%,     at least 70%, at least 80% or at least 90% by weight salt. -   17. A process for plugging a wellbore having a portion extending at     least partially through a caprock composed of salt rock, the process     comprising:     -   introducing a lower foreign-material plug into the wellbore in a         portion of the wellbore at or below the caprock;     -   thereafter, applying a barrier in the wellbore at or above the         caprock; and     -   after applying the barrier, promoting creep such that the salt         rock in the caprock creeps into the wellbore to create a         salt-rock plug in the wellbore to seal across the entire         wellbore, and hence, close off the portion of the wellbore above         the salt-rock plug from fluid flow communication with the         portion of the wellbore below the salt-rock plug. -   18. The process of paragraph 17, wherein the lower foreign-material     plug comprises cement, resin or both. -   19. The process of either paragraph 17 or 18, wherein creep is     promoted by regulating the pressure or temperature below the     barrier. -   20. The process of paragraph 19, wherein the pressure or temperature     are regulated to maintain maximum possible differential stress in     the caprock. -   21. The process of paragraphs 17, 18 or 19, wherein pressure or     temperature is regulated by introducing a fluid into a portion of     the wellbore extending through the caprock. -   22. The process of paragraph 21, wherein pressure is regulated by     changing the density or surface pressure of the fluid. -   23. The process of paragraph 21, wherein temperature is regulated by     changing the temperature of the fluid. -   24. The process of any of paragraphs 20 to 23, further comprising     inserting a pipe through the barrier, and wherein the fluid is     introduced through the pipe into the portion of the wellbore     extending through the caprock. -   25. The process of any of paragraphs 17 to 24, wherein the barrier     is a bridge plug. -   26. The process of any of paragraphs 17 to 25, wherein the step of     promoting creep is continued until a complete wellbore closure     occurs such that the caprock continuum is restored to the state     prior to drilling of the wellbore. -   27. The process of any of paragraphs 17 to 26, wherein after the     step of promoting creep, the process further comprises placing a     plug barrier above the salt-rock plug created by the caprock. -   28. The process of any of paragraphs 17 to 27, wherein prior to     promoting creep, the process comprises introducing an upper     foreign-material plug into the portion of the wellbore extending at     least partially through the caprock, such that during the step of     promoting creep, salt rock creeps radially inward and compress fits     to the foreign-material plug. -   29. The process of paragraph 28, wherein the upper foreign-material     plug comprises cement, resin or both. -   30. The process of any of paragraphs 17 to 29, wherein the caprock     comprises at least 50% by weight salt crystals, or optionally at     least 60%, at least 70%, at least 80% or at least 90% by weight salt     crystals. -   31. The process of any of paragraphs 17 to 29, wherein the caprock     comprises at least 50% by weight salt, or optionally at least 60%,     at least 70%, at least 80% or at least 90% by weight salt.

Therefore, the present compositions and methods are well adapted to attain the ends and advantages mentioned, as well as those that are inherent therein. The particular examples disclosed above are illustrative only, as the present treatment additives and methods may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative examples disclosed above may be altered or modified, and all such variations are considered within the scope and spirit of the present treatment additives and methods. While compositions and methods are described in terms of “comprising,” “containing,” “having,” or “including” various components or steps, the compositions and methods can also, in some examples, “consist essentially of” or “consist of” the various components and steps. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. 

What is claimed is:
 1. A process for plugging a wellbore having a portion extending at least partially through a caprock composed of salt rock, the process comprising: applying a barrier in the wellbore at or above the caprock; and after applying the barrier, promoting creep such that the salt rock in the caprock creeps into the wellbore to create a salt-rock plug in the wellbore to seal across the entire wellbore, and hence, close off the portion of the wellbore above the salt-rock plug from fluid flow communication with the portion of the wellbore below the salt-rock plug.
 2. The process of claim 1, wherein creep is promoted by regulating the pressure or temperature below the barrier.
 3. The process of claim 2, wherein the pressure or temperature are regulated to maintain maximum possible differential stress in the caprock.
 4. The process of claim 2, wherein pressure or temperature is regulated by introducing a fluid into a portion of the wellbore extending through the caprock.
 5. The process of claim 4, further comprising a pipe through the barrier, and wherein the fluid is introduced through the pipe into the portion of the wellbore extending through the caprock.
 6. The process of claim 5, wherein pressure is regulated by changing the density or surface pressure of the fluid.
 7. The process of claim 5, wherein temperature is regulated by changing the temperature of the fluid.
 8. The process of claim 1, wherein after the step of promoting creep, the process further comprises placing a plug barrier throughout the borehole above the salt-rock plug created by the caprock.
 9. The process of claim 1, wherein prior to promoting creep, the process comprises introducing an upper foreign-material plug into the portion of the wellbore extending at least partially through the caprock, such that during the step of promoting creep, salt rock creeps radially inward and compress fits to the foreign-material plug.
 10. The process of claim 9, wherein the upper foreign-material plug comprises cement, resin or both.
 11. The process of claim 1, wherein prior to the promoting of creep, the process further comprising introducing a lower foreign-material wellbore plug into the wellbore in a portion of the wellbore below the caprock.
 12. The process of paragraph 11, wherein the lower foreign-material wellbore plug comprises cement, resin or both.
 13. A process for plugging a wellbore having a portion extending at least partially through a caprock composed of salt rock, the process comprising: introducing a lower foreign-material wellbore plug into the wellbore in a portion of the wellbore below the caprock, and wherein the lower wellbore foreign-material plug comprises cement, resin or both; thereafter, applying a barrier in the wellbore at or above the caprock; and after applying the barrier, promoting creep such that the salt rock in the caprock creeps into the wellbore to create a salt rock plug in the wellbore to seal across the entire wellbore, and hence, close off the portion of the wellbore above the salt-rock plug from fluid flow communication with the portion of the wellbore below the salt-rock plug.
 14. The process of claim 13, wherein creep is promoted by regulating the pressure or temperature below the barrier so as to maintain maximum possible differential stress in the caprock.
 15. The process of claim 13, wherein creep is promoted by regulating the pressure or temperature below the barrier, and wherein pressure or temperature is regulated by introducing a fluid into a portion of the wellbore extending through the caprock.
 16. The process of claim 15, further comprising inserting a pipe through the barrier, and wherein the fluid is introduced through the pipe into the portion of the wellbore extending through the caprock.
 17. The process of claim 13, wherein the step of promoting creep is continued until a complete wellbore closure occurs such that the caprock continuum is restored to the state prior to drilling of the wellbore.
 18. The process of claim 13, wherein after the step of promoting creep, the process further comprises placing a plug barrier above the salt-rock plug created by the caprock.
 19. The process of claim 13, wherein prior to promoting creep, the process comprises introducing an upper foreign-material plug into the portion of the wellbore extending at least partially through the caprock, such that during the step of promoting creep, salt rock creeps radially inward and compress fits to the foreign-material plug.
 20. The process of claim 19, wherein the upper foreign-material plug comprises cement, resin or both. 